A novel motion preserving total disc replacement implant (TDI) is provided. The TDI is particularly designed for implantation into a human patient or other mammal, into the inter-vertebral space between adjacent spinal discs or vertebrae, as a prosthetic replacement for one or more surgically removed discs. The TDI beneficially provides a substantially full and natural post-operative range of motion (ROM).
In the preferred form, the components of the TDI of the present invention are formed from ceramic materials, or biocompatible metals, or a combination thereof, with preferred ultra-low wear ceramic-ceramic or ceramic-metal articulatory components and materials being described in copending U.S. Ser. No. 10/171,376, filed Jun. 13, 2002, and entitled METAL-CERAMIC COMPOSITE ARTICULATION, which is incorporated by reference herein. Such ultra-low wear bearing material or materials have shown impressive mechanical and tribological properties for hip articulations, and may be used in the TDI of the present invention thereby avoiding the problems and disadvantages associated with prior art concepts using metal end plates articulating with a conventional high molecular weight polyethylene (PE) insert.
Spinal disc herniation and the often resultant symptoms of intractable pain, weakness, sensory loss, incontinence and progressive arthritis are among the most common debilitating conditions affecting mankind. If a patient's condition does not improve after conservative treatment, and if clear physical evidence of nerve root or spinal cord compression is apparent, and if correlating radiographic studies (i.e., magnetic resonance imaging (MRI) or X-ray computer tomography (CT) imaging or myelography) confirm the condition, discectomy, or surgical removal of the affected disc is often resorted to. In the United States in 1985, over 250,000 such operations were performed in the lumbar spine and cervical spine.
Statistics suggest that present surgical techniques are likely to result in short-term relief, but do not prevent the progressive deterioration of the patient's condition in the long run. Through better pre-operative procedures and diagnostic studies, long-term patient results have improved somewhat. But it has become clear that unless the removed disc is replaced or the spine is otherwise properly supported, further degeneration of the patient's condition will almost certainly occur.
In the mid-1950's and 60's, Cloward and Smith & Robinson popularized anterior surgical approaches to the cervical spine for the treatment of cervical degenerative disc disease and related disorders of the vertebrae, spinal cord and nerve root; these surgeries involved disc removal followed by interbody fusion with a bone graft. It was noted by Robinsoni that after surgical fusion, osteophyte (bone spur) re-absorption at the fused segment might take place. However, it has become increasingly apparent that unfused vertebral segments at the levels above and below the fused segment degenerate at accelerated rates as a direct result of this fusion. This has led some surgeons to perform discectomy alone, without fusion, by a posterior approach in the neck of some patients. However, as has occurred in surgeries involving the lower back where discectomy without fusion is more common as the initial treatment for disc herniation syndromes, progressive degeneration at the level of disc excision is the rule rather than the exception.
Similarly, in addition to the problems created by disc herniation, traumatic, malignant, infectious and degenerative syndromes of the spine also involve fusion of spine segments. Other procedures include bone grafts and metallic rods, hooks, plates and screws being appended to the patient's anatomy; often they are rigidly and internally fixed. None provide for a patient's return to near-normal functioning. Though these procedures may address the symptoms in the short-term, they can result in progressive degeneration of discs at adjacent levels in the longer term. This is due to the adjacent discs attempting to compensate for the lack of motion of the fused segment. In fact, it is now well recognized that premature degenerative disc disease at the level above and below the excised disc can and does occur. Hence, motion preserving total disc replacements are a promising alternative to spine fusion devices. This next generation of spinal implants in fact, mirror the progression in other articulating joints such as hips and knees: from arthrodesis to arthroplasty.
While long term clinical data are unavailable, the current generation of articulating disc implants typically have metal end plates with a compliant articulating, typically high density polyethylene (PE) insert between them. Compliant inserts are used to enable low friction articulation and also to enable resilient cushioning under load, although no clinical proof exists that shock absorption is necessary. The long history of similar metal/PE articulations for hip and knee protheses indicates that in the long term, PE wear particles are one of the principal causes of implant failures. In hip implants for example, the ultra-high molecular weight polyethylene (PE) particles are released over time from the acetabular linerii, iii, iv. This wear debris is released into the peri-implant tissue and elicits a deleterious biologic reaction, incorporating foreign-body giant cell and macrophage cell responses leading to bone resorption, and eventual implant failure. As a consequence, alternate rigid-on-rigid bearing materials such as alumina-on-alumina ceramic, metal-on-metal, and the recent cobalt-chromium (CoCr) alloy-heavily cross-linked PE have been introduced.
It is instructive to follow the evolution of knee joint implants since they are kinematically analogous to intervertebral discs: they have a similar range of complex motion including sliding in the anterior-posterior (A-P) direction, rotation and bending in the medial-lateral (M-L) direction, and combinations thereof. Early designs had unacceptable failures due to aseptic loosening resulting from poor conformity leading to instability, high contact stresses, and high PE insert wear. Despite lower loads on the knee joint, backside wear of the PE tibial insert resulting from sliding motion was a major cause for concernv. Implant stability was found to be a function of how well the tibial component was fixed. Extensive retrieval analysis of knee implants has indicated that proper fit, fixation and initial stability of the tibial component was critical to achieving clinical success. Stated alternatively, development of an appropriate ingrowth surface to achieve consistent bony fixation over large cancellous regions was critical to implant successvi, vii. More recent knee implant designs have included mobile bearing platforms that allow rotation and A-P translation. The articulations have improved conformity and kinematics, which lead to reduced contact stresses. However new problems arise: bearings without stops can dislocate or spin-out, and bearings with stops can wear as they abut against the mechanical stops that prevent dislocation.viii 
Diagnostic imaging using radiography or MRI is commonly used to assess the presence of spinal disease, determine range of motion or evaluate the patients progress in healing post surgical treatmentix, x. The present generation of total disc replacements use metal end plates which present problems with imaging MRI or in X-Ray-CT imaging, due to the presence of halos and other artifacts.
Finally, an important requirement for total disc implants is that the articulating disc does not protrude or impinge on the spinal cord or nerve roots, which is a concern with compliant materials.xi As has been well established from other articulating joints such as the knee, PE inserts can suffer damage from several modes: creep, pitting, scratching, burnishing, abrasion, delamination and embedded particulates. While there is debate over whether creep or wear is the main cause of dimensional changes in PE insertsxii, xiii, there is little doubt that damage to PE can and does occur over the long term.
Thus, there is a need to develop an alternative to the present generation of disc implants, typically with respect to those designed from metal/PE articulations, and to overcome a number of potential drawbacks:                [a] Long term wear of the articulating PE “disc”, especially in cases where small bony fragments are entrapped between the articulating surfaces,        [b] Osteolysis and subsequent aseptic loosening and instability of the implant as a result of the PE wear debris,        [c] Protrusion of the disc from the disc space due to creep or fatigue related gradual changes in dimensional characteristics, and        [d] Difficulty of diagnostic imaging the intervertebral region because of electromagnetic artifacts, halos and radiographic shadows associated with the metal end plates.        
The proposed TDI design of the present invention is geometrically configured to accommodate a substantially full and natural range of motion, and, in the preferred form, is constructed from an alternate ultra-low wear bearing material that restores anatomic function avoids all the drawbacks of current artificial disc designs.